Secure client connection fails using Postfix/Dovecot guide

So, just worked my way through the EXCELLENT guide for setting up an Postfix/Dovecot email server. Everything appears to work fine and I can mutt in locally and I can mutt in over IMAPS remotely. However, I can't get Thunderbird to connect. I've tried all kinds of different combinations and none works. Any clue what I might be missing?

Note, I do see this in my logs when I last failed to connect from Thunderbird:

Dec 1 22:12:08 myhostname dovecot: imap-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=my remote IP, lip=my linode IP, TLS: SSLread() failed: error:14094418:SSL routines:SSL3READ_BYTES:tlsv1 alert unknown ca: SSL alert number 48, session= I've seen some web forum comments about Thunderbird rejecting my default dovecot.pem cert, but I don't know if that's accurate.